Streams derived from natural gas reservoirs, petroleum or coal, often contain a significant amount of acid gases, for example carbon oxides such as carbon monoxide (CO) and carbon dioxide (CO2), hydrogen sulfide (H2S), sulfur dioxide (SO2), carbon disulfide (CS2), hydrogen cyanide (HCN), carbonyl sulfide (COS), or mercaptans as impurities. These fluid streams may be gas, liquid, or mixtures thereof, for example gases such as natural gas, refinery gas, hydrocarbon gasses from shale pyrolysis, synthesis gas, and the like or liquids such as liquefied petroleum gas (LPG) and natural gas liquids (NGL).
Various compositions and processes for the removal of acid gasses are known and described in the literature. It is well-known to treat gaseous mixtures with aqueous amine solutions to remove these acidic gases. Typically, the aqueous amine solution contacts the gaseous mixture, comprising the acidic gases, counter currently at low temperature or high pressure in an absorber tower. The aqueous amine solution commonly contains an alkanolamine such as triethanolamine (TEA), methyldiethanolamine (MDEA), diethanolamine (DEA), monoethanolamine (MEA), diisopropanolamine (DIPA), or 2-(2-aminoethoxy) ethanol (sometimes referred to as diglycolamine or DGA).
For example, U.S. Pat. No. 4,336,233 discloses the use of a mixture of piperazine and MDEA for the simultaneous removal of CO2 and H2S from gas streams. Piperazine undergoes a rapid reaction with CO2 to form a carbamate whereas MDEA reacts slowly with CO2 because its tertiary alkanolamine structure prevents the formation of a carbamate.
Tertiary alkanolamines, for example MDEA, are often used in combination with an activator such as Piperazine as disclosed in U.S. Pat. No. 4,336,233; U.S. Pat. No. 4,997,630; and U.S. Pat. No. 6,337,059, all of which are incorporated by reference herein in their entirety. Alternatively, DE 1,904,428 discloses the use of a mixture of methylaminoethanol (MMEA, MAE or NMEA) and MDEA for the removal of CO2 and H2S from gas streams and DE 1,474,218 discloses the use of a mixture of diethanolamine (DEA) and MDEA for the removal of CO2 and H2S from gas streams. Thus, MMEA and DEA are known activators of MDEA.
It is often desirable to selectivity remove H2S from gas streams containing both H2S and CO2 thus minimizing the removal of CO2. Solutions of primary and secondary alkanolamine such as MEA, DEA or DGA absorb H2S and CO2 simultaneously removing both acidic gases. United States pipeline specifications for natural gas allow for the stream to contain up to 2 mol % of CO2 meanwhile allowing for up to 4 ppm H2S. Therefore, it could be desirable for natural gas streams containing less than 2 mol % CO2 and more than 4 ppm H2S to be purified with a process minimizing the removal of CO2.
ComponentU.S. Pipeline SpecificationCO2<2mol %H2S<4ppmH2O<120ppmC3+950-1050 Btu/scf dew point −20° C.Total Inerts (N2, He etc)<4mol %
Examples of gas sweetening processes include U.S. Pat. No. 4,471,138 which discloses the use of severely sterically hindered alkanolamines for the selective removal of H2S over CO2. The amines of the invention are perceived to maintain their selectivity for H2S even at high loading of H2S, at high temperature and at low pressure. The commercial usefulness of severely sterically hindered alkanolamine is somewhat limited by their difficult preparation as exemplified by patent publication WO 2005/081778 A2.
GB Patent 2,017,524 discloses the use of tertiary alkanolamine for the selective removal of H2S over CO2. The amines of the invention are perceived to possess high selectivity for H2S in combination with high H2S loading capacity. The selectivity for H2S of tertiary alkanolamines such as methyldiethanolamine (MDEA) has been disclosed as early as 1950 by Frazier and Kohl, Ind. And Eng. Chem., 42, 2288 (1950). The selectivity is attributed to a slow rate of reaction of tertiary alkanolamine with CO2 compared to H2S.
In 1997, Chakma, in the The Canadian Journal of Chemical Engineering, 75, 1997, teaches that the thermal degradation of MDEA leads to the formation of dimethylethanolamine (DMEA) and diethanolamine (DEA). DMEA degrades further to methylaminoethanol (MMEA, MAE or NMEA). Therefore, the thermal degradation of MDEA produces compounds such as MMEA and DEA which are known activators of tertiary alkanolamine such as MDEA towards the removal of CO2. Thus, it is expected that the degree of selectivity of MDEA for H2S over CO2 will decrease overtime as MDEA thermally degrades to MMEA and DEA.
In 2009 and 2010, Carrette and Al. in Energy Procedia 1 2009, 893-900, Ind. Eng. Chem. Res. 2009, 48, 9061-9067 and Ind. Eng. Chem. Res. 2010, 49, 7147-7151 teach that the degradation pathway of MDEA reported by Chakma can be extended to other alkanolamines. These publications also teach that alkanolamine degradation rates are enhanced in the presence of oxygen and that most degradation products are the same in the presence or in the absence of oxygen.
Gas purification processes are widely encountered in refineries. Aqueous alkanolamine solutions are commonly used for this purpose. Among them industry standards evolve from monoethanolamine (MEA) to diethanolamine (DEA). Presently, refiners are converting their DEA processes for more energy efficient methyldiethanolamine (MDEA) processes. Part of the energy improvement comes from the ability of MDEA to remove selectively H2S over CO2 compared to DEA. The most preferred strategy in order to perform the conversion from DEA to MDEA is to keep replenishing the solvent sump with MDEA until extinction of DEA. Shutting down refineries to undertake this task is costly and also builds inefficiency. Therefore, there is a transition period during which the refinery will operate with a solution containing mixture of DEA and MDEA and for which a poor selectivity for H2S over CO2 will be observed.
Therefore, there is a need for a process capable of maintaining the degree of selective removal of H2S over CO2 when using tertiary alkanolamine after the alkanolamine is partially or substantially degraded. There is also a need for a process capable of increasing the rate of conversion of a solution containing a mixture of an activator such as DEA and a tertiary alkanolamine such as MDEA to a tertiary alkanolamine solution containing no activator thus exhibiting a high degree of selective removal of H2S over CO2.